Catalyst for decomposing nitrous oxide and method for performing processes comprising formation of nitrous oxide

ABSTRACT

The present invention relates to a catalyst comprising 0.1-10 mol % Co3-xMxO4, where M is Fe or Al and x=0-2, on a cerium oxide support for decomposition of N2O in gases containing NO. The catalyst may also contain 0.01-2 weight % ZrO2. The invention further comprises a method for performing a process comprising formation of N2O. The N2O containing gas is brought in contact with a catalyst comprising 0.1-10 mol % CO3-xMxO4, where M is Fe or Al and x=0-2, on a cerium oxide support, at 250-1000° C. The method may comprise that ammonia is oxidized in presence of an oxidation catalyst and that the thereby formed gas mixture is brought in contact with the catalyst comprising the cobalt component on cerium oxide support at a temperature of 500-1000° C.

The present invention relates to a catalyst for decomposing nitrousoxide (N₂O) to nitrogen and oxygen at temperatures of 250-1000° C. Theinvention also comprises a method for performing processes comprisingformation of nitrous oxide.

In recent years there has been increasing focus on how to decompose N₂Oas it is an atmospheric ozone depletion gas (greenhouse gas). N₂O willbe formed during the catalytic oxidation of ammonia in connection withnitric acid production and during oxidation of alcohols and ketones, forinstance in connection with the adipic acid production. Also inconnection with the use of N₂O, for instance as an anaesthetic gas, theeffluent N₂O should not be discharged to the atmosphere, but decomposed.

Though N₂O will decompose homogeneously to some extent at hightemperatures, most processes are comprised of the application of varioustypes of catalysts for its decomposition. However, a catalyst which mayfunction well in a certain temperature range and/or gas mixturecontaining N₂O, will not necessarily function for other operatingconditions. The selectivity of the catalyst is also of great importance,especially if the catalyst is applied in connection with ammoniaoxidation, up front of the absorption unit, in a nitric acid plant. Inthat case the catalyst should not decompose the main product, i.e. thenitrogen oxide (NO).

Numerous N₂O decomposition catalysts are known and most of these arebased on various metal oxides such as cerium oxide, cobalt oxide, cupricoxide, chromium oxide, manganese dioxide and nickel oxide as the activecomponent. Furthermore, there are known catalysts based on metal oxideson zeolite carriers and transition metal ion exchanged zeolites.

A catalyst for reducing nitrogen oxide compounds is known from Japaneseapplication JP 48089185. Though this application does not mentionnitrous oxide specifically, its definition also covers this nitrogenoxide. The catalyst contain Co and Ce oxides as its main components. Inan example a mixture of 249 parts cobalt acetate, and 315 parts ceriumacetate was dissolved in water. ZrO₂ was soaked with this solution andpyrolyzed at 900° C. for 5 hours to give a catalyst containing CeO₂ andCo₃O₄ on the surface of the ZrO₂ support.

From the application WO 93/15824 it is known to contact a N₂O containinggas with a catalyst containing nickel oxide plus cobalt oxide on azirconia substrate at 280° C. The ratio nickel oxide to cobalt oxide is0.5-1:3-1. Pure and diluted N₂O containing gases can be treatedaccording to this application.

It is further known from EP 207857B1 that a catalyst comprised of ceriaand 1-20 weight % of at least Al, Si Zr, Th or rare earth metals asoxides. This composition which essentially contains ceria and preferably1-5 weight % of said metal oxides can be used for the synthesis ofmethanol over an extended period without loss of surface area.

From U.S. Pat. No. 4,738,947 It is known that a p-type metal oxide beingdispersed on a refactory oxide such as ceria improves the oxidation ofhydrocarbons and carbon monooxide. It is further claimed that thedispersion with addition of platinum on the refactory oxide results in acatalyst suitable for a catalytic reduction of nitrogen oxide withhydrocarbons and/or carbon monoxide. No reference is made to a catalyticdecomposition of nitrogen oxide without reductants. No example refers tonitrous oxide.

Application WO98/32524 describes an invention related to the catalyticreduction of nitrogen oxide and the use of a catalyst for the reductionof nitrogen oxide and the oxidation of carbon monoxide and hydrocarbons.The essential ingredient is gold which is complexed by a transitionmetal and anchored to an oxide support. No reference is made to acatalytic decomposition of nitrogen oxide without reductants. No examplerefers to nitrous oxide.

Applied Catalysis B: Environmental 13 (1977) 69-79 R. S. Drago et al.describes a catalyzed decomposition of N₂O on metal oxide supports.Decomposition of N₂O using metal oxides supported on silica, magnesiumoxide, calcium oxide and hydrotalcite-like supports were studied. CoOwas found to be a most active catalyst when supported on silica attemperatures of 500° C. The silica supported catalysts were prepared bypore filling the silica support with nitrates of the metals, drying at180° C. and decomposition of the nitrates to oxides at 500° C.

When supporting CoO on MgO a much more active catalyst was attained.However, the activity of the catalyst decreased by calcination at 1000°C. Catalysts calcined at 500° C. gave 99% conversion of N₂O, whilecatalysts calcined at 1000° C. gave 50% conversion of N₂O. Preparationof Co₃Mg₅Al₂(OH)₂OCO₃.y.H₂O “hydrotalcite-like” compound is alsodescribed. This precursor was calcined at 500° C. or 800° C. BETanalysis of Co₂O/2MgO catalysts calcined at 500° C. and 1000° C. showeda surface area of 118 m²/g and 4 m²/g, respectively.

When the catalyst for N₂O decomposition comprises cobalt oxide, asreported in Journal of Chem. Soc. Faraday Trans. 1, 74(7), 1595-603,which studied the structure and activity of Co_(x)Mg_(1-x)Al₂O spinelsolid solutions for use as catalysts in decomposing N₂O, the catalystactivity generally increases when a greater amount of cobalt ions isincorporated into octahedral sites in the structure.

The main object of the present invention was to arrive at a versatile,active and thermally stable catalyst for decomposing N₂O at temperaturesabove 250° C., especially at temperatures of 800-1,000° C.

Another object was that the catalyst should be stable and retain itsactivity for at least a normal cycle, i.e. the length of time betweenchange of the ammonia oxidation catalyst.

A further objective was to produce a catalyst which could be applied athigh space velocities and having a high selectivity for decomposing N₂Owithout decomposing NO.

It was also an objective to arrive at a method for reducing the amountof nitrous oxide from processes comprising formation of nitrous oxide,such as nitric acid processes, adipic acid processes and combustion ofhydrocarbons in vehicle engines.

Another object was to remove nitrous oxide from the tail end gas fromnitric acid plants and other exhaust gases.

The various known N₂O decomposition catalysts were first evaluated withregard to activity and thermal stability. Cobalt oxide was known fromthe literature to possess high activity, at least initially for some gascompositions and at relatively low temperatures. N₂O decomposition inthe tail end gas from nitric acid plants was reported to perform at highactivity by using catalysts prepared from hydrotalcite precursorscontaining cobalt and being lightly calcined, i.e. at 200-500° C. Theinventors therefore started by further investigating this type ofcatalyst for preparation of a process gas catalyst. An essentialrequirement for a new catalyst was that it should be thermally stable atammonia oxidation operating conditions. This means that the catalystmust be active, selective and stable at temperatures of 800-1000° C. andfor the gas mixtures formed during the catalytic ammonia oxidation.

Several cobalt oxide containing precursors were then made and calcinedfor at least 5 hours, at temperatures of about 900° C. Such catalystswere compared with other known catalysts in an initial test of N₂Odecomposition for a gas containing 2932 ppm N₂O and 2574 ppm NO andwhere the ratio NO:N₂O was 0.88 and hourly space velocity, GHSV, was280,000 h−¹. The rate constants for the decomposition were measured at700° C.

These tests confirmed that Co is the essential metal in the Co—Mg—Al(ex-hydrotalcite). The oxides were tested under different GHSV and forthe most active oxides with a conversion too high to give accurate rateconstants. Tests of the thermal stability of a catalyst based oncalcined hydrotalcite containing Co, Mg and Al were performed for 48hours at about 900° C. The tests were performed at GHSV 108,440 h⁻¹ andwith 2932 ppm N₂O, 2575 ppm NO, the rest of the gas mixture being argon.For these types of catalysts the N₂O conversion and their rate constantswere reduced during the test period. The surface area of the catalystwas reduced, from 9.3 down to 1.3 BET m²/g. These results clearlyindicate that the stability of such catalysts is questionable.

The inventors then started to investigate catalysts based on activecomponents on a support, for instance metal oxides like zirconia,alumina, ceria and mixtures thereof. One advantage of these types ofcatalysts will be the material cost reduction if it is possible tosubstantially reduce the amount of the active component in thedecomposition of nitrous oxide.

Firstly, the cobalt-aluminium system was systematically evaluated inlaboratory reactor tests. The composition of the system Co_(3-x)Al_(x)O₄was varied from x=0 to x=2.

The results of laboratory activity data are shown in FIG. 1. It wasobserved that there is an increase in the activity, measured afterapproximately 90 hours operation, as aluminium is added to the spinelstructure. However, when the cobalt/aluminium ratio was less than 1, adecrease in activity was observed. This was surprising as there is acontinuous increase in surface area with increase in the aluminiumcontent. Therefore, in terms of intrinsic reaction rate, it seemedadvantageous to work with a cobalt rich spinel though these materialstend to have a low surface area.

Another oxide system which was also thoroughly examined was theCo_(3-x)Fe_(x)O₄ system, x could vary from 0 to 2. These materials weretested in a laboratory microreactor and the results are shown in FIG. 2.CoFe₂O₄ showed the highest activity. This particular composition may bedescribed as “cobalt stabilised magnetite”.

Although these two types of spinels show high activities, they were notconsidered practical for use as pure phases in a plant for the followingthree reasons: Any catalyst containing a high cobalt content will beprohibitively expensive, all the above active phases, except for thealuminium rich spinel, have low surface areas and they also deactivate,even at the relatively low temperature 800° C. Accordingly, these phasescan only be considered useful if they can be successfully combined withan appropriate support phase. However, many conventional catalystsupports can not be used for this application. The following propertiesare required: The support should be a refractory material, preferablywith a melting point above 1800° C., so that it resists sintering andmaintains a high surface area under the process conditions. Further, thesupport should not react significantly with the active phase resultingin loss of activity and/or selectivity. Finally, the support should bereadily available at a price substantially lower than that of the activephase.

Selection of a suitable support proved to be more complicated thanexpected and it was soon realised that possible combinations of activephase and support material had to be thoroughly evaluated. The firstsupport material examined was magnesium oxide. A pilot activity test ona cobalt aluminate spinel phase, with a nominal composition of Co₂AlO₄was then performed. It was observed that the initial activity of thiscatalyst was good. However, during further testing it was found thatthere was a continuous reduction in performance with time. Detailedanalysis of the catalyst after the pilot test indicated that there wastransport of cobalt from the spinel active phase into the magnesiasupport. Further investigations revealed that the aluminium rich spineland the cobalt-magnesia solid solution exhibit a lower activity than theCo₂AlO₄ spinel and this explained the deactivation. This process willcontinue until the chemical activity of cobalt in the spinel and themagnesia support are the same. Based on these observations and testsmagnesia was excluded as a support for the catalyst to be used in aprocess gas environment.

Another commonly used support material is alumina. However, as with amagnesia support, transport of transition metal from the active phase tothe alumina was found to occur, leading to the formation of alumina richspinels which exhibit a lower intrinsic rate than the cobalt rich spinelor perovskite active phases. Therefore, alumina had to be excluded as arealistic support material. Similar arguments are made against the useof alumino-silicates and alumino-magnesium silicate supports.

Zirconia, ceria and mixtures of these have also been used as supportmaterial in some catalysts for oxidation of carbon monoxide andhydrocarbons, (WO 96/14153), the active catalyst being a noble metal andpossibly also a transition metal. As referred above cerium oxide is alsoused in a catalyst for methanol production. This catalyst contains 1-20%of at least Al, Si Zr or Th. In view of the physical properties of ceriait was decided to investigate this support material further. Thesolubility of cobalt and iron in ceria is low, and the rate of diffusionof these elements into ceria is reported to be very slow, therefore,ceria remained an interesting candidate. Ceria will in most cases be inthe form of CeO₂, but can also be in the form of Ce₂O₃. Laboratory testswith Co₃O₄/CeO₂ were then performed and the activity and stability ofthe catalyst were most promising. Further laboratory tests and pilotplant tests were then performed in order to establish the optimumcomposition of this type of catalyst.

Pure cerium oxide samples, without an active phase component, was alsotested for activity towards N₂O decomposition in the laboratorymicroreactors under standard test conditions. At a temperature of 890°C., conversion of 70% was achieved, compared with conversions of greaterthan 95% for the best supported spinel catalysts. These results indicatean additional advantage or synergy of using cerium oxide as a supportmaterial. The whole area of the catalyst, both the active phase and thesupport material will be contributing to the decomposition of thenitrous oxide. Contrary to cerium oxide, other support materials such asalumina and magnesia were found to be completely inert towards nitrousoxide decomposition.

Ceria supported catalysts could be made in several ways usingconventional catalyst manufacturing methods. Cobalt salts,cobalt-aluminium-salts and cobalt-iron-salts could be precipitated on orimpregnated into cerium oxide powder and the resulting slurry could bedried and calcined. The catalyst particles could then be formed intouseful shape by tableting, compacting, extrusion etc. A high surfacearea of the ceria will be advantageous and as it will be reduced duringcalcination, a ceria with high initial surface area should be used. Atoperating temperature the surface area of the ceria should be largerthan 10 m²/g, preferably larger than 50 m²/g.

The invention is further explained and elucidated in the followingexperiments and corresponding tables and figures.

FIG. 1 shows N₂O conversion of the Co_(3-x)Al_(x)O₄ active phases at890° C. in a laboratory microreactor.

FIG. 2 shows N₂O conversion of the Co_(3-x)Fe_(x)O₄ active phases 890°C. in a laboratory microreactor.

FIG. 3 shows N₂O conversion of Co₃O₄—CeO₂ catalysts in a laboratorymicroreactor.

FIG. 4 shows pilot plant activity data for N₂O conversion of for variouscatalysts at 5 bar, 900° C. and GHSV=66,000 h⁻¹.

FIG. 5 shows data from a laboratory microreactor for N₂O conversionusing various catalysts according to the invention.

FIG. 6 shows effect of catalyst composition on loading on N₂O conversionin a laboratory microreactor.

FIG. 7 shows the effect on N₂O conversion of the addition of ZrO₂ to theCo₂ALO₄/CEO₂ catalyst.

The catalyst according to the invention consists essentially of 0.1-10mol % Co_(3-x)M_(x)O₄, where M is Fe or Al and x=0-2, on a cerium oxidesupport. A preferred catalyst also contains 0.01-2 weight % ZrO₂.

The supported catalyst contains preferably 1-5 mol % of the cobaltcomponent.

The cerium oxide support used in the preparation of the catalyst shouldpreferably have a surface area larger than 10 m²/g, preferably largerthan 50 m²/g at operating temperature.

Preferred cobalt components in the catalyst are, Co₃O₄, Co_(3-x)Al_(x)O₃where x=0.25-2 or Co_(3-x)Fe_(x)O₄ where x=0.25-2.

The main feature of the method according to the invention for performingprocesses comprising formation of N₂O is that the N₂O containing gas isbrought in contact with a catalyst comprising 0.1-10 mol %Co_(3-x)M_(x)O₄, where M is Fe or Al and x=0-2, on a cerium oxidesupport, at 250-1000° C. It is preferred to use a catalyst which alsocontains 0.01-2 weight % ZrO₂.

When the method according to the invention is applied in a nitric acidplant, ammonia is oxidised in presence of an oxidation catalyst and thenthe thereby formed gas mixture is brought in contact with the catalystcomprising the cobalt component on cerium oxide support at a temperatureof 500-1000° C.

The tail end gas from an absorption unit downstream an ammonia oxidationunit, can be brought in contact with the N₂O decomposition catalystcomprising 0.1-10 mol % Co_(3-x)M_(x)O₄, where M is Fe or Al and x=0-2,on a cerium oxide support at a temperature of 250-500° C.

The N₂O containing gas mixture from an adipic acid process can also betreated according to the invention by bringing the said gas in contactwith the N₂O decomposing catalyst comprising 0.1-10 mol %Co_(3-x)M_(x)O₄, where M is Fe or Al and x=0-2, on a cerium oxidesupport at a temperature of 500-800° C.

EXAMPLE 1

This example shows the results from tests performed on laboratory scaleusing catalysts having the respective 1, 5 or 10 mol % concentration ofCo₃O₄ on a ceria support. N₂O conversion % as function of time is shownin FIG. 3. The tests were performed at a pressure of 3 bar, GHSV=560,000h⁻¹ and a gas composition of:

N₂O=1200 ppm

NO=10000 ppm

Oxygen=4%

H₂O=1.7%

the balance being nitrogen.

The tests were performed at 800° C. and 890° C. The results of the testsare shown in FIG. 3. These tests show that the conversion of N₂O wasvery high, about 98%. The stability of the catalyst was also promising.Best results were obtained when the catalyst contained 5 mol % of thecobalt component.

EXAMPLE 2

The catalysts used in the tests of example 1 were then tested in a pilotplant having ammonia oxidation conditions. Further, the tests comprisedinvestigations of unsupported Co₂AlO₄ catalyst and a catalyst beingCo₂AlO₄ on MgO support The N₂O conversion catalyst was placed rightbelow the ammonia oxidation catalyst and platinum recovery gauze's. Thetests were performed at the following standard conditions: Pressure 5bar, temperature 900° C., GHSV 55.000 h⁻¹-110,000 h⁻¹. The ascomposition was:

N₂O=1200-1400 ppm, NO=10%, Oxygen=4%, H₂O=16% and the balance beingnitrogen (plus Ar, CO₂, etc. from air).

The results of these tests are shown in FIG. 4 and show that for thebest catalyst the N₂O conversion was about 95% after 100 days on stream.The NO decomposition was well below 0.5% which was considered anacceptable level.

FIG. 4 further shows that the unsupported Co₂AlO₄ catalyst and the sameactive phase on MgO both lost most of their activity after a few days ofoperation.

EXAMPLE 3

This example show the results from tests performed for 90 hours in alaboratory microreactor. The operating conditions were as in example 1and the tests were run at temperatures of 800° C. and 890° C. Theresults are shown in FIG. 5 which shows that the cobalt aluminate onceria is a more stable catalyst than cobalt oxide on ceria.

EXAMPLE 4

This example shows the effect on catalyst composition and loading on N₂Oconversion in a laboratory reactor. The operating conditions were as inExample 1. The tests were run at 890° C. For all the catalysts accordingto the invention the best results are obtained at a catalyst loading ofabout 2 mol %, but for some high activity is achieved already at verylow loading as illustrated in FIG. 6. N₂O conversion of more than 95%can be obtained at very low catalyst loading and even at as much as 10mol %, but nothing seems to be gained by increasing the catalyst loadingabove 5 mol %. The catalyst loading will accordingly also depend onpractical and economical evaluations.

EXAMPLE 5

This example shows the effect of ZrO₂ addition to the performance of theCO₂AlO₄/CeO₂ catalyst installed in an ammonia oxidation pilot plant. Thecatalyst was prepared by mixing the ingredients as in previous examplesplus adding 0.01 weight %-2 weight % fine ZrO₂ powder (particle size 1-3μm). The tests were performed under the same standard conditions as forExample 2. In FIG. 7 the results are presented for an addition of 0.22,0.28 and 0.90 weight % ZrO₂ compared to catalysts without ZrO₂. Theoptimum concentration in this case was 0.2 weight %. The effect ofadding ZrO₂ is reduced degradation of catalyst activity over time.

The inventors have by the present invention succeeded in arriving at aversatile, active and thermally stable N₂O decomposition catalyst. Thecatalysts according to the invention can be applied at a widetemperature range and will also be stable at varying gas composition.Presence of water, which often is a problem, for instance in connectionwith catalyst for exhaust from car engines, have been found to be noserious problem for these catalysts. Accordingly, the new catalysts canbe applied for decomposing N₂O from processes comprising formation ofN₂O. The catalysts are especially useful in connection with nitric acidproduction as the N₂O can be decomposed in the process gas formed afterammonia oxidation without significant destruction of NO and also be usedin the tail gas from the subsequent absorber unit.

1. A catalyst for the decomposition of N₂O at temperatures of 250-1000°C. wherein the catalyst comprises 0.1-10 mol % Co_(3-x)Al_(x)O₄, where0.25≦x≦1, on a cerium oxide support, wherein the catalyst decomposesN₂O, and wherein the catalyst has a high selectivity for decomposing N₂Owithout significant destruction of NO.
 2. The catalyst according toclaim 1, wherein the supported catalyst contains 1-5 mol % of the cobaltcomponent.
 3. The catalyst according to claim 1, wherein the surfacearea of the cerium oxide support used in the preparation of the catalystis larger than 10 m²/g at operating temperature.
 4. The catalystaccording to claim 1, wherein the catalyst further comprises 0.01-2weight % ZrO₂.
 5. A method for decomposing N₂O, without significantdestruction of NO, comprising contacting an N₂O containing gas with acatalyst comprising 0.1-10 mol % Co_(3-x)Al_(x)O₄, where 0.25≦x≦1, on acerium oxide support, at a temperature in the range of 250-1000° C., inan oxidizing environment.
 6. The method according to claim 5, whereinthe catalyst further comprises 0.01-2 weight % ZrO₂.
 7. The methodaccording to claim 5, comprising oxidizing ammonia in the presence of anoxidation catalyst and contacting the thereby formed gas mixture withthe catalyst at a temperature in the range of 500-1000° C.
 8. The methodaccording to claim 5, comprising contacting a tail end gas from anabsorption unit downstream an ammonia oxidation unit with the catalystat a temperature in the range of 250-500° C.
 9. The method according toclaim 5, comprising contacting an N₂O containing gas mixture from anadipic acid process with the catalyst at a temperature in the range of500-800° C.
 10. The catalyst according to claim 2, wherein the catalystfurther comprises 0.01-2 weight % ZrO₂.
 11. The catalyst according toclaim 3, wherein the catalyst further comprises 0.01-2 weight % ZrO₂.12. The method according to claim 6, comprising oxidizing ammonia in thepresence of an oxidation catalyst and contacting the thereby formed gasmixture with the catalyst at a temperature in the range of 500-1000° C.13. The method according to claim 6, comprising contacting a tail endgas from an absorption unit downstream an ammonia oxidation unit withthe catalyst at a temperature in the range of 250-500° C.
 14. The methodaccording to claim 6, comprising contacting an N₂O containing gasmixture from an adipic acid process with the catalyst at a temperaturein the range of 500-800° C.
 15. The catalyst according to claim 3,wherein the surface area of the cerium oxide support is larger than 50m²/g.